Patients in outpatient clinics and wards often get nervous when they see “joint effusion, synovitis, and joint capsule swelling” on the MRI report, and some see “joint capsule swelling” as “cyst”. In fact, all of the above are manifestations of joint effusion, which is caused by inflammation and injury of the joint, resulting in exudation and subsequent appearance of fluid, such as rheumatoid arthritis and osteoarthritis, and meniscus and ligament injury of the knee joint and rotator cuff injury of the shoulder joint. “Once the inflammation or injury is resolved, the joint effusion, synovitis, and joint capsule swelling will improve or disappear. There are also some cases of joint effusion after minor trauma, and there is no obvious damage on the MRI, which means that the damage is not serious and will get better after rest, so there is no need to be nervous.